The present invention relates to a sawtooth waveform generator suitable for use in a convergence correction circuit for a display apparatus such as a multi-scan display (supporting multiple frequencies) or the like.
FIG. 1 is a circuit diagram showing a conventional sawtooth waveform generator.
Referring to FIG. 1, reference numeral 1 denotes an operational amplifier, 4 denotes a capacitor, 5 and 8 denote resistors, 9 denotes a MOSFET, and 10 denotes a diode.
In the sawtooth generator shown in FIG. 1, the operational amplifier 1 forms part of a constant current supply circuit together with a resistor 5 and the MOSFET 9 is used as a switch, and a reset pulse is applied to the gate of the MOSFET 9 through the diode 10. First, the capacitor 4 is charged by a constant current from the operational amplifier 1, and then, the electric charges on the capacitor 4 are discharged owing to the MOSFET 9 held on during the reset period of the reset pulse, and thus a sawtooth voltage is thereby generated as an output +E.sub.0.
When the sawtooth waveform generator is used, for example, in a convergence correction circuit for a display apparatus, the generated sawtooth wave is squared and a parabolic voltage is thereby obtained, and such a parabolic voltage is used as the correction signal for convergence correction. Further, the n-th power of the same is used as a correction signal of geometric distortion.
FIG. 2A to FIG. 2C and FIG. 3A to FIG. 3C are waveform diagrams showing voltage waveforms at principal portions in FIG. 1 and other waveforms related thereto.
FIG. 2A and FIG. 3A show voltage waveforms of reset pulses, FIG. 2B and FIG. 3B show waveforms of A.C. components of output voltages +E.sub.0, and FIG. 2C and FIG. 3C show square waveforms of the waveforms in FIG. 2B and FIG. 3B.
In the conventional sawtooth waveform generator shown in FIG. 1, when a reset pulse as shown in FIG. 2A is input thereto, a sawtooth voltage as shown in FIG. 2B (the waveform of FIG. 2B shows the A.C. component of the sawtooth voltage) is generated.
When the sawtooth waveform generator is used, for example, in a convergence correction circuit for a display apparatus, as described above, the generated sawtooth voltage is squared for obtaining a correction signal for convergence correction. Then, a parabolic voltage as shown in FIG. 2C is obtained.
While no problem arises as long as the above mentioned display apparatus remains an ordinary display, the following problems arise when the same is a multi-scan display.
As well known, a multi-scan display must support a wide variety of frequencies. Hence, the convergence circuit therefor must also be able to support multiple frequencies. Therefore, the sawtooth generator must be able to generate sawtooth voltages at various frequencies.
Then, when the charge period in the reset pulse is varied with the pulse width (i.e., the reset period) kept constant in order that a sawtooth voltage at a frequency different from that shown in FIG. 2B is generated in a conventional sawtooth generator as shown in FIG. 1, the reset pulse becomes, for example, as shown in FIG. 3A.
If such reset pulse is input, a sawtooth voltage as shown in FIG. 3B (the waveform in FIG. 3B shows the A.C. component) is generated and if, then, the generated sawtooth voltage is squared for obtaining a correction signal for convergence correction, a parabolic voltage as shown in FIG. 3C is obtained.
When the charge period of the reset pulse is varied with the pulse width kept constant, as seen from comparison of FIGS. 3A, 3B with FIGS. 2A, 2B, the generated sawtooth voltage suffers a change in its average level because the relation of duties between the reset period and the charge period is changed. More specifically, while the average level in the case of FIG. 2B is slightly below the level in the middle between the maximum value and minimum value of the sawtooth wave, the average level in the case of FIG. 3B is considerably below the level in the middle between the maximum value and minimum value of the sawtooth wave.
Further, as seen from comparison of FIG. 3C with FIG. 2C, a large phase error is produced in the parabolic voltage obtained by squaring the sawtooth voltage with the above described change in the average level. When the trough portion of the parabolic voltage (which temporally corresponds to the point of intersection of the sawtooth voltage and the average level during the charge period) is designated as point A, this point A in the case of FIG. 2C is located slightly to the left of the point in the middle of the charge period, but the point A in the case of FIG. 3C is located considerably to the left of the midpoint of the charge period. When the phases .phi. from the point at the trailing edge of the reset pulse to the point A are compared, it is known that the phase .phi. in FIG. 3C is smaller than the phase .phi. in FIG. 2C and thus a phase error is produced in FIG. 3C.
The phase error produced in the parabolic voltage due to the arrangement to vary the frequency becomes a factor deteriorating the characteristic of a multi-scan display to follow frequency changes. Therefore, such a sawtooth waveform generator as shown in FIG. 1 has been unable to be used in a convergence circuit for a multi-scan display.